# Aerodynamics Questions and Answers – Supersonic Wind Tunnels

This set of Aerodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Supersonic Wind Tunnels”.

1. Which nozzle is used in supersonic wind tunnel?
a) Convergent
b) Divergent
c) Convergent – Divergent
d) Conical

Explanation: Supersonic wind tunnel produces flow of Mach number above 1. This is achieved using convergent – divergent nozzle. This selection is based on area – velocity relation, according to which the subsonic flow accelerates in the convergent section to sonic speed after which the speed is further accelerated in the divergent section.

2. Which of these is not a part of supersonic wind tunnel?
a) Nozzle
b) Test section
c) Diffuser
d) Test model

Explanation: Supersonic wind tunnel mainly comprises of three components – convergent – divergent nozzle, test section which is constant area duct and a diffuser to further slow down the speed to low subsonic speed. Test model is not a part of the wind tunnel and is inserted inside to take measurements such as lift, drag.

3. How many throat/throats are there in a supersonic wind tunnel?
a) 1
b) 2
c) 3
d) 4

Explanation: In case of supersonic wind tunnels, there are two throats present. One is the nozzle throat where the sonic speed is achieved and the second throat is the diffuser throat where supersonic incoming flow is isentropically compressed to Mach 1.

4. Normal shock diffuser is less efficient than the oblique shock diffuser.
a) True
b) False

Explanation: The goal of the diffuser is to reduce the flow velocity with small total pressure loss. If we reduce the incoming supersonic flow through a series of oblique shock followed by a weak normal shock wave, leads to lower total pressure loss compared to reducing the incoming supersonic flow to subsonic using one strong normal shock.

5. In reality what happens due to the interaction of the oblique shock waves with the boundary layer on the walls of the diffuser?
a) Higher total pressure loss
b) Reduction of speed
c) Total pressure increase
d) Reduced skin friction drag

Explanation: In real scenario far from hypothetical nature of the diffuser, the oblique shock waves interact with the viscous boundary layer of the diffuser wall. This leads to the separation of the boundary layer thereby increasing the total pressure losses. One thing to remember is that the aim of the diffuser is to reduce the flow velocity by low total pressure losses, but the boundary layer separation prevents that from happening.

6. The nozzle throat area is same as the diffuser throat area.
a) True
b) False

Explanation: If we consider the nozzle throat area as section 1, then the mass flow rate through the section is $$\dot {m_1}$$ = ρ1u1A1t. The mass flow rate through th diffuser throat area is $$\dot {m_2}$$ = ρ2u2A2t. Since there’s steady flow in side the wind tunnel, thus the mass flow rate remains same.
$$\dot {m_1} = \dot {m_2}$$
ρ1u1A1t = ρ2 u2A1t
But due to the presence of shock waves inside the diffuser, the density and the flow velocity are not same. (ρ1≠ρ2, u1≠u2). This means that the throat areas are also not same.

7. What is the relation between the nozzle A1t and diffuser throat area A2t in the supersonic wind tunnel?
a) At2 = At1
b) At2 > At1
c) At2 < At1
d) At2 At1 = 1

Explanation: The relation between the throat areas in terms of total pressure is given by:
$$\frac {A_{t2}}{A_{t1}} = \frac {p_{01}}{p_{02}}$$
Since in the diffuser section, the total pressure drops across oblique shock wave, thus p02 < p01. Hence the throat area of the diffuser is larger than the throat area of the nozzle.

8. In which scenario is the throat area of both the nozzle and diffuser the same?
a) Ideal isentropic diffuser
c) Ideal isobaric diffuser
d) Ideal Isochoric diffuser

Explanation: In case of ideal isentropic diffuser the total pressure remains constant. Thus from the relation between the throat area and total pressure we see that p01 = p02 = constant. Hence the throat areas At2 = At1. It is important to note that ideal isentropic diffuser is a hypothetical case and is not possible to achieve in real life.
$$\frac {A_{t2}}{A_{t1}} = \frac {p_{01}}{p_{02}}$$

9. What is the ratio of throat area of diffuser to nozzle in supersonic wind tunnel with flow at Mach 2.7?
a) 0.4236
b) 0.8338
c) 2.36
d) 1.199

Explanation: Given M = 2.7
If we assume that there’s a normal shock present at the entrance of the diffuser, then using the gas table we can find the ratio of total pressures $$\frac {p_{01}}{p_{02}}$$.
For M = 2.7, $$\frac {p_{02}}{p_{01}}$$ = 0.4236
Using the relation beween the throat areas and total pressure we get
$$\frac {A_{t2}}{A_{t1}} = \frac {p_{01}}{p_{02}} = \frac {1}{0.4236}$$ = 2.36

10. In how many categories is the supersonic wind tunnel classified into?
a) 2
b) 3
c) 4
d) 6

Explanation: Supersonic wind tunnel is broadly classified into two categories – Intermittent wind tunnel and continuous wind tunnel. Out of these two, the intermittent wind tunnel is more commonly used due to simpler design, faster start and more power available to start the wind tunnel.

Sanfoundry Global Education & Learning Series – Aerodynamics.

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